I. Field of the Disclosure
The technology of the disclosure relates generally to accessing circuits over a multi-drop bus.
II. Background
Modern electronic devices (e.g., computers, laptops, smartphones, etc.) all require a large amount of on-board memory for application processing and data storage needs. One type of on-board memory is known as a synchronous dynamic random access memory (SDRAM). Advancement of SDRAM technology has led to a class of high-density, high-throughput double data rate (DDR) SDRAM. The latest versions of DDR SDRAM include personal computer (PC) DDR-3, low-power (LP) DDR-3, PCDDR-4, and LPDDR-4. DDR SDRAM integrated circuits (ICs) are often packaged into an integrated memory module commonly referred to as a dual inline memory module (DIMM). Multiple DIMMs are usually needed to provide the large amount of on-board memory required by memory-consuming electronic devices.
By design, a multi-drop memory bus is configured to provide connections to multiple DIMMs. In particular, a memory controller communicates with each of the DIMMs over the memory bus, with a DIMM being associated with each drop on the multi-drop memory bus. Depending on bus topology, electrical characteristics experienced at the target DIMM may vary significantly depending on which DIMM is the target DIMM. That is, impedance changes created by different geometries, reflections associated with stubs on the bus, and other incongruities may all contribute to a bus that has a first signal profile when signaling to a first DIMM and a second signal profile when signaling to a second DIMM.
While the latest versions of DDR SDRAM provide dynamic on-die termination, which allows the memory controller to reconfigure memory terminations independently depending on to which device the memory controller is currently writing, empirical evidence suggests that equalization methods suffer from the requirement for co-optimization for all potential connections, leading to a compromised solution, which is sub-optimal for any particular connection, albeit generally acceptable for all connections.